1.

Scope

The OSI Connection-mode Network Service (CONS) is defined in terms of a set of primitive actions and events and associated parameters. For a protocol to support this service, there must be a mapping between the abstract primitives and parameters of the CONS and the real elements of the protocol. For the X.25 Packet Layer Protocol (PLP), the main body of this International Standard provides such a mapping for the X.25/PLP-1984 using Virtual Calls.

This International Standard alos provides a mapping of the CONS primitives and parameters to the X.25/PLP-1980 plus an SNDCP (Annex A). These mappings apply to the X.25 VC service. In addition, the method of selecting the appropriate mapping, if any, for different combinations of end systems and Network Layer relay systems implementing one or more of the mappings is defined (Annex B).

For the PVC service for both the X.25/PLP-1984 and the X.25/PLP- 1980, a mapping of CONS primitives and parameters to the X.25/PLP is given in Annex C.

This International Standard specifies two sets of procedures from which three classes of implementation are described. The requirements of these procedures are applicable both to end system operation and to half the operation of a Network Layer r elay. Where relay operation is concerned, the two halves of the relay may be the same or different classes of implementation.

This International Standard specifies the requirements to be met by a System-1984 implementation. Implementations in this class are d esigned to operate directly and efficiently with other System-1984 implementations, including cases of operation across an X.25 (1984) subnetwork.

This International Standard also specifies the procedures to be operated by a System-1980 implementation. Implementations in this class are designed to operate directly with other System- 1980 implementations, including cases of operation across any form of X.25 subnetwork, but will operate less efficiently than System-1984 implementations.

This International Standard also specifies the requirements to be met by a Compatible implementation. Implementations in this class are designed to operate directly with all other implementation classes, including cases of operation across any type of X.25 subnetwork. They make efficient use of X.25(1984) when placed in this environment.

The X.25/PLP is usually regarded as operating between an end system (i.e., a ""Data Terminal Equipment"" in X.25 terminology) and a packet-switched public data subnetwork. However, the X.25/PLP can also be used in other environments to provide the OSI CONS. Examples of such other uses include (a) an end system connected to an X.25 packet-switched private data subnetwork; (b) an end system connected to a local area network; (c) direct connection or circuit-switched connection (including connection across a circuit-switched data subnetwork) of two end systems without an intervening packet-switched public data subnetwork; (d) an end system connected to an Integrated Services Digital Network.